1999 blazer blows cold air replace thermostats and flush out h.c.

It could be the heater control valve make sure the vacuum line is connected to the valve and that it is working properly. If it is not replace the valve. Hope this helps.

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Sounds like your thermostat is on it's way out. Do a good cooling system flush then replace thermostat.

The problem was a vacuum line leak. Replaced the line.

Start by flushing your coolant system and replacing your thermostat. You may later find that you have a clogged heater core.

your core could be clogged. you can flush it by removing both heater hoses coming from firewall and spaing the garden hose in one and then switching to the other until clean.

repair shops will not flush core when flushing coolant unless you ask them to.

sounds like the controller went bad use temp checker see what your temp is at inlet of heater core from hose

Have the cooling system flushed or remove the hoses from the firewall(when cool) and blow out heater core with compressed air-clogged heater core..

if youre driving down the road with decent heat and all of a sudden it gets cooler, and the engine temperature remains normal, i would strongly suggest flushing out the heater core at the firewall in engine compartment, very common problem due to the dex-cool antefreeze used be GM.